The present invention relates generally to antennas, and more particularly to a compact antenna system with a reduced directional pattern in the backward hemisphere.
Global navigation satellite systems (GNSSs) can determine locations with high accuracy. Currently deployed GNSSs include the United States Global Positioning System (GPS) and the Russian GLONASS. Other GNSSs, such as the European GALILEO system, are under development. GNSSs are used in a wide range of applications, such as surveying, geology, and mapping.
In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the navigation receiver. A critical component of a GNSS is the receiver antenna. Key properties of the antenna are bandwidth, multipath rejection, size, and weight.
High-accuracy navigation receivers typically process signals from two frequency bands. Two common frequency bands are a low-frequency band in the range of 1164-1300 MHz and a high-frequency band in the range of 1525-1610 MHz.
For portable navigation receivers, antennas with light weight and compact size are desirable. In surveying applications, for example, an antenna is mounted on a surveying pole. The dimensions of the antenna should be sufficiently small to accommodate mounting on a standard surveying pole. The weight of the antenna should also be small enough to ensure easy handling of the pole-mounted assembly: if the weight is excessive, the center-of-gravity is raised too high, and the pole-mounted assembly is unwieldy.
Navigation receivers achieve the highest accuracy when they receive only the direct, line-of-sight, radio signals from the satellites. Navigation receivers, however, typically operate in environments in which the radio signals reflect off environmental surfaces, such as earth and water, and objects, such as buildings, towers, and vehicles. Reflected signals that are detected by the navigation receiver are referred to as multipath signals. Multipath signals reduce the accuracy with which the position of the navigation receiver can be determined.
Antennas that reject or suppress the reception of multipath signals, therefore, are desirable. PCT International Publication No. WO 2004/027920 (I. Soutiaguine et al.), for example, describes an antenna system that operates over a wide bandwidth and reduces multipath reception of GPS signals. FIG. 1 shows a cross-sectional view of the antenna system 100, which includes two micropatch antennas. The micropatch antenna 120 is a directly-excited active antenna; the micropatch antenna 130 is a passive antenna excited by the field of the active micropatch antenna 120.
The active micropatch antenna 120 includes the ground plane 102 and the radiating patch 104. The ground plane 102 and the radiating patch 104 are separated by the dielectric substrate 106. The radiating patch 104 is actively driven by the excitation pin 108.
The passive micropatch antenna 130 includes the ground plane 102 and the radiating patch 110. The ground plane 102 and the radiating patch 110 are separated by the dielectric substrate 112. The radiating patch 110 has no excitation pin and is driven by the field from the active micropatch antenna 120.
The fields of the micropatch antenna 120 and the micropatch antenna 130 are mutually suppressed in the backward hemisphere; consequently, the level of the directional pattern in the backward hemisphere is reduced. The antenna system 100, however, suppresses the multipath signal only within a narrow bandwidth; and the stacked antenna construction has the further disadvantages of heavy weight and large dimensions.
The operating bandwidth for the antenna system 100 is dependent on the distance between the radiating patch 104 and the ground plane 102. To reduce the antenna dimensions and expand the directional pattern in the forward hemisphere, the space between the radiating patch 104 and the ground plane 102 is filled with the dielectric substrate 106. Suitable dielectric materials over the operating frequency bands, however, have a high density; consequently, the weight of the antenna system increases significantly.
As discussed above, in surveying applications, an antenna is mounted on a surveying pole. Examples of pole-mounted GPS antennas are given in European Patent Application Publication No. EP 1503176 (F. Ohtomo et al.) and United States Patent Application Publication No. 20100211314 (Zhukov et al.). In both instances, the antenna extends considerably beyond the pole (particularly along lateral dimensions), and the configuration of outer mounting elements reduces the stability of the overall pole-mounted assembly.
A GNSS antenna with compact size, low weight, and high multipath rejection is therefore advantageous. An antenna that can operate over dual frequency bands and that can be readily mounted onto a standard surveying pole, while maintaining ease of handling, is further advantageous.